Usually, an optical fiber is made of silica glass or plastics. Among them, the silica glass-made optical fiber has good light transmission and practically widely used, for example, in the field of telecommunication. On the other hand, the plastic optical fiber has lower light transmission than the silica glass-made one, but it is light and has better flexibility and processability. By making use of these properties, it is proposed to use the plastic optical fiber in various fields, for example, in a field of short distance telecommunication or as a light guide or a sensor. Some of these applications require good heat resistance of the optical fiber. For example, the optical fiber to be used in optical data link of an automobile should resist a temperature as high as 100.degree. to 120.degree. C. generated from an engine housing. The conventional plastic optical fiber, however, cannot resist a temperature higher than 80.degree. C. since it has a core made of polystyrene or polymethyl methacrylate. This is because such the conventional plastic fiber shrinks at a high temperature and its light transmission is deteriorated and further, at a temperature higher than 100.degree. C., the fiber shrinks so greatly that not only the light transmission is deteriorated but also the fiber itself is broken, which means the complete failure of light transmission.
To prevent the shrinkage of the plastic optical fiber by heat in use, it may be preheat-treated. Although the preheat-treating can decrease the shrinkage of the fiber, it is made less flexible so that it tends to be more easily broken by vibration or bending. Therefore, the preheat-treating of the plastic optical fiber is not practically employed to prevent its shrinkage.
It is known that electron beam radiation improves thermal characteristics of some polymers such as polyvinyl chloride, polyethylene and an ethylene/vinyl acetate copolymer. Such polymers are roughly classified into two classes, namely a cross-linking type and a main chain cleavage type. The cross linking type includes polyvinyl chloride, the ethylene/vinyl acetate copolymer and polyvinylidene fluoride, and the main chain cleavage type includes polymethyl methacrylate. Therefore, it is expected that when the optical fibers comprising polymethyl methacrylate were subjected to electron radiation, the main chain of the polymer would be cleaved and the thermal and mechanical characteristics would be deteriorated.
It has now been found that certain specific polymers comprising alkyl methacrylate, however, behave as cross linking type polymers and the electron radiation will raises glass transition temperatures of the polymers to afford good materials for the plastic optical fiber.